Wave Skewness Research Articles

Coastal processes and dune stability: Insights from wave transmission and runup modeling

Dunes not only serve as natural landscapes but also act as crucial natural barriers protecting coastlines from storm surges. However, the stability of coastlines and dunes is further affected by climate change-induced sea level rise and increased storm activity. An effective evaluation of dune stability necessitates the collection of parameters such as wave height, changes in dune crest elevation, and dune erosion rates. This research employs wave flumes and high-speed camera technology in laboratory settings to observe and simulate coastal dynamics under overwash and collision regimes. The study investigates wave propagation under both regimes using the XBeach model. In the overwash regime, the model slightly overestimates the values of infragravity waves, whereas its accuracy improved under the collision regime. Moreover, in the overwash regime, higher wave skewness results in more sediment transport to the shore, exhibiting a linear relationship between sediment erosion volume and shoreline retreat distance. Through comparison with time series wave runup data obtained by cameras and traditional predictive formulas, the study validates the applicability of the formula proposed by Stockdon as a predictive tool for wave runup in this experiment. To evaluate dune stability, the study introduces the dimensionless overwash threshold parameter Cs, which is based on wave runup and dune crest elevation, to distinguish between dunes in a collision regime or an overwash regime. These findings help identify dune stability patterns, aiding in early detection of coastal erosion and assisting authorities in ecosystem management.

Laboratory investigations of wave-induced transport of plastic debris over a rippled bottom

Laboratory experiments are conducted in a wave flume to investigate the effect of water waves on the transport of plastic pellets over a rippled bottom. The horizontal velocities of plastic debris are analyzed over the rippled bottom for different wave conditions and plastic elements with different properties. Laboratory investigations determined the characteristic transport patterns of wave-induced plastic debris with a density of ∼2.0g/cm3 moving along the rippled bottom. In the first, swing-type motion, the grains move only in the ripple trough with velocities lower than 0.10 m/s. For sliding-type movement, the grains move along the entire rippled surface with velocities in the range of 0.10–0.13 m/s. For higher velocities in the range of 0.15–0.20 m/s, a saltation-type motion becomes dominant. The results show that plastic grains may move up to 2–3 cm above the ripple crest depending on hydrodynamic conditions. The analysis shows that for velocity-skewed flows, sliding-type motion and onshore transport dominate. For acceleration-skewed flows, saltation-type motion and offshore transport dominate, which is attributed to higher boundary layer thickness and phase lag effects. The analysis of the relationship between the particle Reynolds number and the thickness of the turbulent boundary layer reveals that for values of Rep≥1000 and a boundary layer thicknessδ≥50 mm saltation-type motion becomes dominant. The direction of transport is affected not only by the density of the sediment and the wave skewness coefficients but also by the dimensions of the bottom ripples. The laboratory investigations also provide insight into the hydrodynamic conditions affecting the transport of plastic debris along the bottom covered with ripples in oscillating nonlinear water flows.

Parameterisation and evolution of non-breaking wave nonlinearity over flexible vegetation

Wave nonlinearity significantly influences wave attenuation and can cause sediment transport imbalances in vegetated zones, impacting shoreline stability and ecosystem-based coastal defences. Despite its importance, the parameterisation and mechanisms of wave nonlinearity evolution over flexible vegetation remain unclear. This study examines the mechanisms behind non-breaking wave nonlinearity evolution over flexible vegetation and provides parameterisation of wave nonlinearity, based on experimental data collected by Anderson and Smith (2014). The study derives a new set of empirical formulae for predicting wave nonlinearity in Spartina alterniflora, which considers Ursell number, relative vegetation width, vegetation submergence, and vegetation density. The predictions made using these formulae are in good agreement with the measurements. The results indicate a decrease in wave skewness across the span of vegetation, whereas wave asymmetry increases until it reaches a maximum before decreasing. The findings suggest a preferential dissipation of high harmonics compared to low harmonics in the vegetation zone. Additionally, bispectrum analysis reveals that vegetation enhances the difference interaction while suppressing the sum interaction between wave components. The findings enable improvements in our understanding of wave nonlinearity in vegetation, providing better predictive capabilities for wave attenuation and sediment transport, which are crucial for the design and optimization of coastal defence systems.

Laboratory observation of nonlinear wave shapes due to spatial varying opposing currents

Physical experiments were conducted to examine the influence of adverse currents on the propagation of shallower water waves and their impact on the evolution of nonlinear wave shapes. Irregular waves, characterized by varying initial peak periods and amplitudes, were generated in a physical wave flume equipped with a bottom slope of 1/20. Three groups of spatially varying opposing currents were generated in the flume and interacted with the aforementioned wave trains. Experimental results confirm that strong opposing currents can significantly intensify local wave nonlinearity, thereby further influencing the deformation characteristics of waves. In contrast, a weak opposing current (without wave-blocking) has a negligible effect. Bicoherence analysis revealed that the degree of phase coupling among triads of waves increased with an elevated current velocity before wave-blocking. Nevertheless, during partial wave-blocking, the degree of phase coupling was seemingly weakened by an increase in opposing current. Moreover, the influence of a co-existing current in the formation of extreme waves was recognized. The study confirms that adverse currents notably affect extreme wave steepness and extreme wave skewness, with a negligible impact on extreme wave asymmetry. As a result, empirical formulas modified by current effects are presented, describing certain key nonlinear wave shapes as functions of the local Ursell number.

Morphological Changes in Storm Hinnamnor and the Numerical Modeling of Overwash

Constant changes occur in coastal areas over different timescales, requiring observation and modeling. Specifically, modeling morphological changes resulting from short-term events, such as storms, is of great importance in coastal management. Parameter calibration is necessary to achieve more accurate simulations of process-based models that focus on specific locations and event characteristics. In this study, the XBeach depth-averaged model was adopted to simulate subaerial data pre- and post-storms, and overwash phenomena were observed using the data acquired through unmanned aerial vehicles. The parameters used for the model calibration included those proposed in previous studies. However, an emphasis was placed on calibrating the parameters related to sediment transport that were directly associated with overwash and deposition. Specifically, the parameters corresponding to the waveform parameters, wave skewness, and wave asymmetry were either integrated or separated to enable an adequate representation of the deposition resulting from overwash events. The performance and sensitivity of the model to changes in volume were assessed. Overall, the waveform parameters exhibit significant sensitivity to volume changes, forming the basis for calibrating the deposition effects caused by overwashing. These results are expected to assist in the more effective selection and calibration of parameters for simulating sediment deposition due to overwash events.

Non-intrusive measurements of wave nonlinearity over movable bed of sediment with different beach slopes

A series of laboratory experiments were undertaken to qualitatively investigate the evolution of wave nonlinearity over a movable bed of sediment with five different beach slopes under regular waves in a medium-scale wave flume. An innovative non-intrusive data collecting system, which mainly consists of three side-looking high-speed cameras, was developed to collect high-resolution and synchronous data on free-surface water elevation of waves and bed level changes without causing any disturbances to wave motions and the movable bed of sediment. On analyzing the collected experimental data, it is found that regular waves become nonlinear when they propagate to the shoaling zone and start breaking, and linear wave theory is quite accurate for calculating wave parameters such as orbital velocity with the correlation coefficient r2 = 0.8–0.95 before the waves break, but becomes less accurate after the waves break or are in the breaking zone with the smaller correlation coefficient r2 = 0.4–0.6. Four parameters, wave skewness Skη, asymmetry Ayη, Ursell number Ur, and Rocha number NP0, are introduced to describe the wave nonlinearity, of which Skη and Ayη are found to be of largest amplitudes at the wave breaking point and then start to decrease in the breaking zone and are almost unchanged for different beach slopes, while Ur further increases in the breaking zone and exceed the first larger value as waves approach to shoreline, but NP0 is almost linearly proportional to wave orbital velocity amplitude and quite sensitive to beach slope. The location of sandbar is found close to the wave breaking point in the wave flume and may be also considered as the point where wave nonlinearity becomes important for sediment transport in the surf zone, and the linear wave theory becomes less accurate.

Numerical Modelling of Beach Profile Evolution with and without an Artificial Reef

With the recent development from grey infrastructures to green infrastructures, artificial reefs become more popular in coastal protection projects. To investigate the responses of beach profile evolution to the presence of an artificial reef, a non-hydrostatic model is established. Both hydrodynamic and morphodynamic evolution for the beach with and without an artificial reef are compared under regular wave conditions. In addition, the protected beach profile evolution by an artificial reef is discussed under irregular wave conditions. Three key parameters in non-hydrostatic simulation are considered for sensitivity analysis, including maximum wave steepness criterium (maxbrsteep), water depth factor (depthscale), and equilibrium sediment concentration factor (sedcal). The numerical results under regular wave conditions indicate that the artificial reef enhances wave attenuation by inducing wave breaking. In addition, the artificial reef reduces local flow velocity and offshore sediment transport by 51%, therefore decrease the total erosion by 53%. Over the artificial reef, wave skewness and asymmetry go through a drastic change. Under irregular wave conditions, short waves contribute to the wave energy mainly and reflection-induced standing wave effects decline considerably. It demonstrates that the artificial reef can protect the beach from regular and irregular waves by reducing erosion and offshore transport of suspended sediments. Moreover, in the wave breaking area, the increase of maximum wave steepness criterium may give arise to the wave height. The morphological evolution is more sensitive to water depth factor than equilibrium sediment concentration factor, because the former is a controlling factor for beach profile characteristics while the latter forms the sandbar varying irregularly in shape.

Alongshore variability in berm and sandbar migration patterns on a highly dynamic beach

Sediment exchange between the shoreface and the foreshore is investigated through the analysis of sandbar and berm migration cycles at La Misión beach in Baja California, Mexico. In general, nearshore sandbars migrate onshore during low-energy wave conditions merging with the shoreline to become intertidal sandbars that later contribute to berm rebuilding. However, significant alongshore differences were observed in the sandbar welding and berm rebuilding process, which can be attributed to alongshore wave skewness and near-surfzone flow variations. The analysis of topographic and bathymetric surveys collected monthly over a 5-year period revealed larger shoreline retreat and longer recovery times in the southern beach compared to the north. This was related to the inability of sandbars to weld to the intertidal beach during spring and summer, occurring only during extended periods of low-energy waves. In the southern beach, the berm formation process exhibited less dependence on incoming wave energy, persisting during mild-energy winters, and reforming in the presence of sandbars. Hydrodynamic measurements indicated that the southern part of the beach experienced relatively more intense net offshore flows (up to −0.2 ms−1) due to undertow, and possibly related to the presence of persistent rip currents. In contrast, the berm in the northern beach followed a seasonal erosion and rebuilding process associated with sandbar migrations driven by variations in incoming wave energy and cross-shore sediment transport. This was characterized by larger near-bed wave velocity skewness compared to the south. Net flows in the north were predominantly onshore, reaching up to 0.1 ms−1 during lower-energy periods with oblique wave incidence. This study reveals the presence of considerably distinct sandbar and berm migration cycles along a relatively short beach of 2.2 km long. It also highlights how such variability can be related to the morphodynamic feedback with sandbars affecting hydrodynamics, particularly near-bed wave skewness and near-surfzone flow variations, which in turn influence sediment transport and morphological evolution.

EFFECTS OF WAVE SKEWNESS AND ASYMMETRY ON THE EVOLUTION OF FIRE ISLAND, NEW YORK

Bedload transport of sediment by waves and currents is one of the key physical processes that affect the evolution of coasts, nearshore areas, and the engineering practices there. Wave skewness and asymmetry, both of which increase as waves shoal, result in a net bedload sediment flux over a wave cycle. The impacts of this mechanism on large-scale coastal and shoreline change are investigated in this study, using field observations and Coupled Ocean Atmosphere Wave Sediment Transport (COAWST), a hydrodynamic process-based numerical modeling system (Warner et al., 2010). The study site is Fire Island, New York, located at the Atlantic Coast of the USA, with a focus on the persistent shoreline shape, at the western half of this 50-km-long barrier island, that has been hypothesized to be linked to the sand deposits at the shoreface.

Effects of currents on nonlinear wave transformation in a reef-lagoon-channel system

Wave nonlinearity can drive the near-bed sediment transport and nutrient delivery on coral reefs. Previous studies have primarily concentrated on the wave nonlinearity evolution for a specific cross-shore profile and have scarcely taken the effects of currents into consideration. To better understand the nonlinear wave transformation on reef coasts, the wave height distribution and wave nonlinearity evolution are analyzed based on 3D reef-lagoon-channel experiments in a wave basin. By taking the current velocity into consideration, the Glukhovskiy distribution function is improved in calculating the probability of large wave height in the sequence. Wave skewness and asymmetry on the reef and in the channel are parameterized with the Ursell number considering the Doppler effect induced by opposing or following currents. The accuracies for estimating wave skewness and asymmetry are improved by 39% and 66%, respectively. The influence of Doppler effect on wave shapes on the reef flat and in the channel varies significantly with the incident wave heights and reef submergences. Furthermore, it is also noticed that the reduction of local water depth caused by the propagation of infragravity wave troughs increases short-wave skewness at individual wave scale on the middle of reef flat, while its effect on short-wave asymmetry is not significant.

Development, calibration and validation of a phase-averaged model for cross-shore sediment transport and morphodynamics on a barred beach

Simulating cross-shore sediment transport and associated sandbar migration is still a challenging task for phase-averaged coastal morphological models. Numerical studies have mostly relied on beach morphology prediction for calibration and validation, without examining in much detail the underlying hydrodynamics, sediment concentrations and transport rates. This paper reports on a new three-dimensional coastal morphodynamic model based on the hydrodynamic model of Zheng et al. (2017), combined with an advection-diffusion type suspended sediment transport model and the extended SANTOSS near-bed sediment transport formula of Van der A et al. (2013), to represent the key cross-shore transport mechanisms. The model is are calibrated based on comprehensive measurements from a large-scale laboratory experiment involving regular plunging breaking waves over an evolving sandbar, covering detailed comparisons on hydrodynamics, sediment suspension, transport rates, and bed level evolution. Separate validation using large scale wave flume experiments were also conducted to confirm the model's performance on different conditions. Good agreements are obtained between measurements and model results, which demonstrates the model's ability to reproduce cross-shore sediment transport processes under breaking waves correctly, given that the appropriate parameterizations for intra-wave processes are included. Model results also reveal the onshore near-bed transport is related to wave-induced near-bed streaming, wave skewness and asymmetry, and bed slope effects at different locations across the beach surface. Wave breaking-induced turbulence enhances the near-bed transport within the bed boundary layer which needs to be taken into account in order to achieve good model prediction skills.

A new expression for free stream velocity with skewness and asymmetry

A new expression for skewed and asymmetric free stream velocities (near-bed wave orbital velocities at the edge of the wave boundary layer) is established in this paper. A comparison of the new expression with others is examined, and the new expression is found to be more satisfactorily fitted to wave orbital velocities with comparatively strong skewness and weak asymmetry than those presented by Abreu et al. (2010) and Malarkey and Davies (2012). Methods for determining the parameters in the new expression based on the Ursell number Ur are presented, and consequently, two empirical formulae are recommended for estimating bedload and suspended load sand transport rates related to wave skewness, respectively. Reasonability of the new expression for irregular waves is discussed by comparing the predicted time averaged quantities to those calculated by measured data, and good agreement is found.

Inter-comparison of wave skewness and asymmetry estimation using wavelet, Fourier and statistical methods

The traditional definition of wavelet bispectrum does not give the quantification of the bispectral content of an area of frequency–frequency space and bicoherence for signals exhibiting rapidly changing amplitudes and limited length can be misleading. To overcome these drawbacks, this research applies a newly defined quantitative wavelet bispectrum to the analysis of wave skewness and asymmetry over typical coastal topographies. Comparing with statistical methods and the Fourier-based bispectrum, the quantitative wavelet bispectrum with a lognormal mother wavelet is validated and can be used to calculate wave skewness and asymmetry. Parameters that can influence the wavelet transform is analyzed. For laboratory cases, number of voices nv=8 is adequate to discretize the interested frequency band with sufficient resolution. The influence of wavelet peak frequency is analyzed. The magnitude of wavelet derived skewness (Sk) and asymmetry (As) increases with an increasing f0. Based on the analysis results, procedures to carry out wavelet transform to analyze wave records is suggested.

Effect of Wave Skewness and Sediment Particle Size on Sediment Transport Due to Combined Wave–Current Seabed Boundary Layer Streaming

Effect of Wave Skewness and Sediment Particle Size on Sediment Transport Due to Combined Wave–Current Seabed Boundary Layer Streaming

Geometry of deep and intermediate water breaking waves influenced by wind speed and direction

Wind effects on the evolution of a breaking wave group due to dispersive focusing are investigated using a two-phase flow Navier–Stokes solver. The Navier–Stokes equations are solved for both air and water, with the air–water interface captured by the Volume of Fluid method and the turbulence by the Smagorinsky subgrid-scale stress model. The two-phase model is first validated with the experimental measurements with and without following wind action. The following wind delays the breaking and shifts the breaking location downstream, and vice versa for the opposing wind. The wind-induced drift current is mainly responsible for these shifts of breaking time and location. The shift of breaking location and time is approximately linearly proportional to the wind speed (wind induced drift current) under weak winds, but the shift saturates under strong winds. The direct wind forcing, on the other hand, plays an increasingly larger role in the wave breaking process in the presence of stronger wind. It was found that the strong following wind forcing enhances the initiation of wave breaking, while the strong opposing wind forcing may change breaking type or suppress wave breaking of large intensity, such as plunging breaking. Accordingly, the wave shape at breaking onset is altered considerably under strong winds. The following wind increases the maximum wave height and wave skewness slightly. However, the opposing wind may also increase the maximum wave height initially because of the wind drift current induced upwave refocusing of the wave group. Eventually, stronger opposing wind decreases the wave height and wave skewness.

Influence of artificial sandbar on nonlinear wave transformation: Experimental investigation and parameterizations

Wave nonlinearity is a main driving force for near-bed sediment transport, and thus affects the beach evolution. Accurate prediction of wave nonlinearity is an essential need for the design of beach nourishment and restoration projects. So far, wave nonlinearity evolution patterns on the nourished beaches with abrupt changes of bottom slopes and water depth are not fully understood. This paper presents experimental results of nonlinear wave transformation across the submerged artificial sandbar. It can be found that the artificial sandbar intensifies wave shoaling processes and increases the magnitudes of the wave nonlinearity. Wave skewness and asymmetry are parameterized and both dependent on Ursell number and bottom slopes. By taking bottom slopes into consideration, the prediction accuracy for wave skewness is improved 17% with dataset measured in this study and 21%–65% with an independent dataset, compared with a common-used formula also calibrated on a steep low-crest breakwater without considering the bottom slope. Wave skewness decreases with increasing Ursell number in the inner surf zone, indicating the artificial sandbar influences its shoreward wave nonlinearities through the wave history. Finally, it is found that wave triads interactions can be enhanced by the steep seaward slope and small water depth of the artificial sandbar.

Effect of surface wave skewness on near-bed sediment transport velocity

The paper presents a two-dimensional model, based on the Lagrangian description of sand grain dynamics, for the estimation of mean velocities of sediment particles driven by the oscillatory motion of water generated by non-linear gravitational surface waves of the Stokes type. The model was verified against experimental results obtained in a wave flume, by comparing its predictions with the sediment velocities measured by applying a PIV (particle image velocimetry) technique. The model was used to calculate near-bed sediment transport velocities generated by surface waves of the parameters encountered in real sea conditions, with the purpose to investigate the effect of the wave velocity skewness on the near-bed sediment net transport rates. The results of numerical simulations, carried out for a range of wave parameters and sand grain sizes, showed that normalized sediment transport velocities vary approximately linearly with an index defining the wave skewness, and are practically independent of sediment grain diameters. Based on the laboratory measurements, also approximate formulae relating a bed ripple asymmetry index with the surface wave skewness index were derived.

Small-scale bedforms and associated sediment transport in a macro-tidal lower shoreface

Ripples and small-scale bedforms are ubiquitous in shallow water environments under the combined action of currents and waves. Small scale processes linked to their formation and migration are interconnected with sediment transport at larger scales (e.g. tens of metres to kilometres), both resulting in and being affected by large scale sediment transport and geomorphological evolution. The lower shoreface provides a key link between coasts and continental shelves, but the contribution of ripples and small-scale bedforms to sediment transport in this region has yet to be fully addressed. This work presents a study of sediment dynamic processes on the lower shoreface in the presence of small-scale bedforms. Observations were made during the winter of 2017 on the lower shoreface of Perranporth Beach, which is in the south west of the UK and exposed to Atlantic waves. The analysis of morphological expressions and the variability of ripples under waves, currents and wave-current conditions are assessed. Ripple morphology and associated dynamics are analysed for their potential contribution to the exchange of sediment between the lower and the upper shoreface. In the present study it was observed that even though ripples were evolving depending on the wave-current forcing, little ripple migration was observed due to low wave skewness. The implication is that ripple migration and bedload transport are only a small contribution to onshore sediment transport under low to moderate energy conditions. However, during more energetic conditions, ripples were washed out and the wave skewness increased, resulting in onshore sediment transport under a sheet flow regime. This suggests that ripple formation and migration can have little impact on the cross-shore supply of sediment from the lower shoreface to the upper shoreface and that more energetic wave conditions are required to significantly transport sediment towards the beach.

Wind-induced changes to surface gravity wave shape in shallow water

Abstract

Experimental Observation on Beach Evolution Process with Presence of Artificial Submerged Sand Bar and Reef

For observation on the influence mechanism of environmentally and aesthetically friendly artificial submerged sand bars and reefs in a process-based way, a set of experiments was conducted in a 50 m-long flume to reproduce the cross-shore beach morphodynamic process under four irregular wave conditions. The beach behavior is characterized by the scarp (indicating erosion) and the breaker bar (indicating deposition), respectively, and the scarp location can be formulated as a linear equation regarding the natural exponential of the duration time. Overall, main conclusions are: (1) the cross-shore structure of significant wave height and set-up is mainly determined by the artificial reef (AR); (2) the cross-shore distribution of wave skewness, asymmetry, and undertow (indicating shoaling and breaking) is more affected by the artificial submerged sand bar (ASB); (3) the ASB deforms and loses its sand as it attenuates incident waves, which leads to a complex sediment transport pattern; (4) the scarp retreat is related to the beach state, which can be changed by the AR and the ASB; (5) the AR, the ASB, and their combination decrease wave attack on the beach. In conclusion, this study proves positive effects of the AR and the ASB in beach protection through their process-based influences on beach behaviors and beach states for erosive waves.